Largest two digit prime natural number

Hi my sisters and brothers can tell me the answer because i don’t understand the question

Given the following linear function sketch the graph of the function and find the domain and range. ƒ(x) = -5x + 4

Sally deposited money into a savings account paying 4% simple interest per year.the first year,she earned $75 in interest.how much interest will she earn during the following year?